The invention pertains to the field of bimorphs in general, and more particularly, to the field of bimorph based light modulators.
Bimorphs are not new devices. Basically, a bimorph is a device manufactured with two strips of piezoelectric film which are fastened together and which have electrodes allowing electrical fields of the proper polarity to be applied through the film to cause an electrostrictive effect to occur. This electrostrictive effect changes the dimensions of the film of the two different strips in such a way that the bimorph bends.
Bimorphs have been used in the prior art to modulate light by using the bimorph to bend into and out of the path of a light beam. The physical occlusion of the light path by the bimorph interrupts the light beam and therefore modulates the light in accordance with the timing and intensity of the electrical fields applied to the bimorph. Bimorphs have also been used in the prior art to interrupt the light path at the focal point between two lenses, and have been used to trip the shutter mechanisms of cameras when a sufficient amount of light for proper exposure has been received. It is known in the prior art to attach a right angle shutter to a bimorph and to allow the bimorph to bend into and out of the path of light through an aperture in a mask plate. It is also known to use bimorphs in display elements such as seven segment displays where instead of using light-emitting diodes for each of the segments in the display, a bimorph painted with a distinctive color is used to activate each display segment. It is also known in the prior art to doubly support the bimorph with a fixed fulcrum somewhere in the middle of the length of the bimorph and a movable fulcrum on one end thereby leaving one end of the bimorph free to move in response to the applied electric fields.
It is also known in the prior art to use piezo film for its piezoelectric property in order to manufacture transducers. That it, when piezo film is subjected to mechanical stress, a voltage can be generated which can be sensed to signal the occurrence of an event causing the mechanical stress.
It is also known in the prior art to use bimorphs in conjunction with a Mach-Zehnder optical interferometer to implement an optical phase shifter in fiber optic sensor systems.
Other workers in the art have used bimorph light beam deflectors wherein a mirror is placed at the end of a bimorph and a laser beam is directed onto the mirror such that the angle of reflection is altered by the bending of the bimorph. Still other workers in the art have used bimorphs to implement a mechanical multiplexer for fiber optic switching of light from one input fiber to either of several output fibers.
There are certain problems which arise from the use of bimorph cantilevered beams for the interrupting of light paths. For one, the cantilevered beam has its own mechanical resonant frequency. When such a beam is excited by a narrow pulse or a step function, the beam bends and mechanical resonance or vibration often occurs, causing the free end of the beam to vibrate. If the vibration causes any portion of the bimorph or the shutter to move into and out of a light path, errors in the desired average light flux will occur. Further, when driving such a bimorph at high frequencies, the acceleration of the beam and its velocity is high. If a mechanical stop is used to limit the travel of the beam, the bimorph can hit the mechanical stop and bounce. Such bouncing action is called chatter and also causes errors in the average light flux passing the bimorph since the light flux is calculated under ideal conditions where no bounce occurs.
It is useful to use bimorph light modulators in large area displays where each bimorph modulates the average light flux emerging from a particular pixel location or from one color component of a pixel in a three-primary-color pixel. Such displays have advantages over conventional secondary emission displays in that a light source of any desired intensity may be used to supply the input light which may then be modulated using the individual bimorphs in accordance with scene information to control the gray scale light intensity of each pixel. The advantage of such an arrangement is that high contrast and good visibility during high ambient light conditions can be achieved. That is, the light emerging from the face of such a display can be made much more intense than the light emerging from secondary emission displays such as cathode ray tubes (CRTs) and television type screens since phosphor light emission is limited in intensity. In contrast, a display using bimorphs is not limited in the intensity of the light at each pixel location by the physical nature of any secondary emission type material such as phosphor. Further, such a display can be made very large since the light from the source can be directed to very large numbers of pixels by optical channels such as fiber optic light guides, and there is no need for deflecting an electron beam to raster scan the entire display.
Use of bimorphs to control the pixel light intensities in a large scale display requires very accurate correspondence between the electronic signal which encodes the desired amount of light at each pixel location and the actual average light flux which is gated through to that location by the bimorph. Further, video displays require vast quantities of data to be handled in very short times if the display is to be compatible with NTSC and PAL television signals. Thus, each bimorph must be able to operate at a fairly high frequency and accurately control the average light flux passing through the light channel controlled by that bimorph.
The bimorph structures taught in the prior art could not be used for application to such a large scale display. For one thing, the structures taught in the prior art suffer from resonance and chatter problems which would degrade the accuracy and repeatability in controlling the average light flux passing through the light channel controlled by each bimorph. Further, the bimorph structures taught in the prior art would suffer from electrostatic pinning problems which would degrade the ability of the bimorph to operate at high frequencies necessary to handle NTSC and PAL television signals. The bimorph structures taught in the prior art would also be unreliable since no means is taught for preventing the electrostrictive dimensional changes of the bimorph film from occurring at the locations of electrical contacts. Thus, the electrical contacts can be rendered intermittent or be caused to fail altogether by the mechanical stresses induced when the film changes dimensions under the contact locations.
Further, the bimorphs of the prior art are generally glued together with glues which render the assembly of the bimorph difficult. It is important to be able to glue the two film strips together without bubbles, wrinkles or other stress in the film which could cause curl in the final structure. With the types of glue taught in the prior art, only a limited amount of time is available before the glue sets to adjust the two films and eliminate wrinkles, curls and stresses. This would make assembly and registration of the two films and removal of wrinkles, bubbles and other stress-producing artifacts more difficult.
Further, the prior art does not teach a method of registering the bimorph and shutter with a light path to insure that complete occlusion of the light path will occur when the bimorph is in the "off" position. Since bimorph film is extremely thin and is made of polymer film, there is often curl in the final bimorph product which varies from one bimorph to another. It is important to be able to register all the bimorph shutter controlling elements at the outset to insure that when all the bimorphs are in the "off" position, the shutters for each bimorph completely occlude all light paths, and none are in a prestressed state which is different from the prestressed state of the others. If this is not the case, each bimorph will act differently in response to the same signal.
Accordingly, a need has arisen for a bimorph light modulator for use in implementing large scale displays which can operate at high enough frequency to be compatible with television signals and which can control very high intensity light such that the display is usable in high ambient light conditions with good contrast and visibility. Further, such bimorphs must be relatively easy to assemble, and must be reliable and accurate in terms of the repeatability of the light intensity modulation which may be achieved.